1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device means a general device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). As semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, a transistor whose active layer uses an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of lower than 1018/cm3 is disclosed (see Patent Document 1).
A transistor including an oxide semiconductor can operate at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon; however, even when the gate potential is the same as the source potential (that is, a state where the gate voltage is 0 V), current (leakage current) flows between the source and the drain (so-called normally on).
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2006-165528